Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-10-17
2001-09-25
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S647000
Reexamination Certificate
active
06293126
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for separating air.
BACKGROUND OF THE INVENTION
The separation of air by rectification is very well known indeed. Rectification is a method in which mass exchange is effected between a descending stream of liquid and an ascending stream of vapour such that the ascending stream of vapour is enriched in a more volatile component (nitrogen) of the mixture to be separated and the descending stream of liquid is enriched in a less volatile component (oxygen) of the mixture to be separated.
In particular, it is known to separate air which has been cooled in a main heat exchanger in an arrangement of rectification columns comprising a higher pressure column and a lower pressure column. An initial separation is performed in the higher pressure column and as a result an oxygen-enriched liquid fraction is formed at its bottom and a nitrogen vapour fraction at its top. The nitrogen vapour fraction is condensed. A part of the condensate provides reflux for the higher pressure column and another part of the condensate provides reflux for the lower pressure column. A stream of oxygen-enriched liquid is withdrawn from the higher pressure column and is passed through an expansion device, normally a valve, into the lower pressure column. Here it is separated into oxygen and nitrogen fractions which may be pure or impure. Nitrogen and oxygen products are typically withdrawn from the lower pressure column and are returned through the main heat exchanger in countercurrent heat exchange with the first stream of compressed air. It is conventional to sub-cool the oxygen-enriched liquid stream upstream of the expansion device by indirect heat exchange with a nitrogen gaseous product stream withdrawn from the lower pressure column. Such sub-cooling reduces the amount of flash gas that is formed on expansion of the oxygen-enriched liquid stream. As a result, higher reflux ratios can be obtained in those regions of the lower pressure column below that at which the oxygen-enriched liquid stream is introduced, thereby facilitating the efficient operation of the lower pressure column. In addition, the sub-cooling has the effect of raising the temperature of the nitrogen product stream passing through the sub-cooler. This tends to have the benefit of reducing temperature differences in the main heat exchanger between air streams being cooled and product streams being warmed, and thereby leads to more efficient heat exchange. Nonetheless, the addition of a sub-cooler does add to the complexity of the air separation plant.
EP-A-0 848 220 shows, for example, in FIG. 8 an air separation plant in which the oxygen-enriched liquid stream taken from the higher pressure column is sub-cooled in the main heat exchanger. U.S. Pat. No. 5,275,004 discloses employing the main heat exchanger to perform the function of the reboiler-condenser that normally places the top of the higher pressure column in heat exchange relationship with the bottom of the lower pressure column. It is further disclosed in U.S. Pat. No. 5,275,004 that where the process comprises sub-cooling a liquid process stream in a sub-cooler, the sub-cooler's heat exchange service can be performed in the main heat exchanger.
It is an aim of the present invention to provide a method that enables a simplification of an air separation plant to be made without necessitating an undue loss of operating efficiency.
SUMMARY OF THE INVENTION
According to the present invention there is a method of separating air, wherein a first stream of compressed air is cooled in a heat exchanger and downstream of the cooling is rectified in an arrangement of rectification columns comprising a higher pressure column and a lower pressure column; a stream of oxygen-enriched liquid is withdrawn from the higher pressure column, is expanded and is introduced into the lower pressure column; a second stream of compressed air is cooled at a higher pressure than the first stream of compressed air; the first and second streams of compressed air are cooled in indirect countercurrent heat exchange with a gaseous nitrogen stream taken from the lower pressure column; the first stream of compressed air passes out of heat exchange relationship with the gaseous nitrogen stream at a higher temperature than the second stream; at least part of the second stream of air downstream of its heat exchange with the nitrogen stream is expanded and is introduced into the lower pressure column; and the stream of oxygen-enriched liquid passes essentially isenthalpically from the higher pressure column to its expansion, wherein the entire cooling of the second stream of compressed air from 0° C. is performed in the same heat exchanger as the cooling of the first stream of compressed air, and the second stream of air passes out of heat exchange with the nitrogen stream at a temperature at least 5 K lower than the bubble point temperature of air at the pressure prevailing at the inlet for the first stream of compressed air to the higher pressure column.
Because the stream of oxygen-enriched liquid passes isenthalpically of the first expansion device, it does not pass through a sub-cooler. The omission of a sub-cooler for the oxygen-enriched liquid stream facilitates the fabrication of the air separation plant because the conduit that conducts the oxygen-enriched liquid from the higher pressure column to the lower pressure column can be located relatively close to the columns and does not have to pass through a conventional sub-cooler separate from the main heat exchanger, or through the main heat exchanger itself in the manner of the corresponding conduit shown in FIG. 8 of EP-A-0 848 220. Further, the disadvantageous effect on the operation of the lower pressure column by not sub-cooling the stream of oxygen-enriched liquid is largely mitigated by the cooling of the second stream of compressed air to a lower temperature than the first stream of air. Preferably, the second stream of air passes out of heat exchange with the nitrogen stream at a temperature at least 5 K and more preferably at least 10 K less than the bubble point temperature of air at the pressure of the inlet to the higher pressure column. If supplied at a pressure less than its critical pressure, the second stream of compressed air is liquefied and sub-cooled in its indirect heat exchange with the nitrogen stream. Moreover, since many air separation processes make use of liquid air, little additional cost will typically be added by the sub-cooling of this air. Indeed, the entire cooling of the second stream of compressed air from 0° C. is preferably effected in the same heat exchanger as that in which the first stream of compressed air is cooled.
The first and second streams of compressed air are preferably also cooled by indirect heat exchange with a stream of oxygen withdrawn from the lower pressure column. The purity of the oxygen may be selected in accordance with the requirements of any process to which the oxygen is supplied.
Particularly efficient heat exchange can be achieved if the stream of oxygen is withdrawn in liquid state from the lower pressure column and is raised in pressure upstream of its heat exchange with the first and second streams of compressed air.
Typically the arrangement of rectification columns comprises a double rectification column in which an upper region of the higher pressure column is placed in heat exchange relationship with a lower region of the lower pressure column by a reboiler-condenser. In such examples of the method and plant according to the invention that employ a double rectification column a stream of liquid nitrogen is preferably withdrawn from the reboiler-condenser is sub-cooled, is expanded through a third expansion device, and is introduced into the lower pressure column as reflux. This additional sub-cooling is preferably performed in indirect heat exchange with the said gaseous nitrogen stream. Thus, the need to have a separate sub-cooler for the liquid nitrogen is obviated. Preferably, the gaseous nitrogen stream passes essentially
Cheung Wan Yee
Doerrler William
Pace Salvatore P.
The BOC Group plc
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